Wrocław (pronounced like vro-swavh) is a city about 260km north of Krakow, and using Polskibus it takes about 3 hours each way. The city is similar to Krakow (has the same look and feel, and has a big main square), and is definitely worth visiting if you are in Krakow for more than a few days.

It is a pretty city (like Krakow), with a fair few things to do, but it is definitely not as touristy as Krakow.